(HealthNewsDigest.com) - Inactivity rather than overeating could be driving the surge in Americans' obesity, according to a study by a team of Stanford University School of Medicine researchers.Examining national health survey results from 1988 through 2010, the researchers found huge increases in both obesity and inactivity, but not in the overall number of calories consumed.

"What struck us the most was just how dramatic the change in leisure-time physical activity was," said Uri Ladabaum, MD, associate professor of gastroenterology and lead author of the study. "Although we cannot draw conclusions about cause and effect from our study, our findings support the notion that exercise and physical activity are important determinants of the trends in obesity."

The study will appear in the August issue of The American Journal of Medicine. It's also available online now in a draft form.

The researchers analyzed data from the National Health and Nutrition Examination Survey, a long-term project of the Centers for Disease Control and Prevention that collects information from surveys and from physical examinations to assess Americans' health. The researchers considered survey results from 17,430 participants from 1988 through 1994 and from approximately 5,000 participants each year from 1995 through 2010.

Survey participants recorded the frequency, duration and intensity of their exercise within the previous month. The team defined "ideal" exercise as more than 150 minutes a week of moderate exercise or more than 75 minutes a week of vigorous exercise.

The research highlights the correlation between obesity and sedentary lifestyles, but because it is an observational study, it does not address the possible causal link between inactivity and weight gain.

The percentage of women reporting no physical activity jumped from 19 percent to 52 percent between 1988 and 2010; the percentage of inactive men rose from 11 percent to 43 percent over the same period.

Obesity also increased, climbing from 25 to 35 percent in women and from 20 to 35 percent in men.

Surprisingly, however, the number of calories consumed per day did not change significantly. Nonetheless, diet remains a proven and important component of health, and participants may have been tempted to under-report how much they ate, Ladabaum said.

Both obesity and abdominal girth, which the team analyzed independently, contribute to a variety of well-documented conditions, such as cancer and cardiovascular disease, as well as increased mortality.

In 2010, 61 percent of women and 42 percent of men had too much belly fat, up from 46 percent and 29 percent in 1988. In addition, the waists of even normal-weight women swelled between 1988 and 2010, the study showed.

Ladabaum noted that the study did not follow one group of participants over that 22-year span; instead, the data came from different samples in each survey cycle. But the samples are constructed to be representative of the population.

Obesity is a complex, multifaceted problem linked to a variety of societal factors, Hannley said in an interview. "There are societal and economic forces at work that we must address," she said. "Take, for example, the struggle of single mothers who are trying to balance work and child care. They may lack the time or resources to exercise. We shouldn't assume that people are just lazy. Their lives might be overwhelming to them."

"It's going to take widespread change," she said. "We shouldn't just tell patients they need to work out. We need to work with communities, employers and local governments to enable healthy lifestyles by ensuring that there are safe spaces to exercise that are cheap or free."

Other Stanford co-authors of the study are Ajitha Mannalithara, PhD, social science research associate; Parvathi Myer, MD, a former postdoctoral scholar who is now at Kaiser Permanente, and Gurkirpal Singh, MD, adjunct professor of gastroenterology.

The study was funded by the National Institutes of Health (grant T32DK007056).

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